1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a method of allocating a Walsh code resource.
2. Discussion of the Related Art
Generally, a wireless communication environment is a channel varied due to positions and mobility of terminals, which requires an adaptation method for the variable channel. A method for efficiently using a wireless channel and transmitting data at a high speed in the wireless channel environment varied with the passage of time is divided into a method for accepting variations of a channel and adaptively changing a modulation type and a method for varying channel coding.
In order to reduce an error rate by adding redundant information data by using channel coding, it is essential to increase data transmitted to the wireless channel. Since the wireless communication environment is a channel varied due to positions and mobility of terminals, it may be good or poor.
Therefore, when the wireless communication environment is good, a lot of information data are transmitted using a high code rate coding that reduces redundancy, thereby increasing a data rate. When the wireless communication environment is poor, low code rate coding which increases redundancy to reduce errors is employed to be resistant to noise and decrease a data rate.
On the other hand, when the wireless communication environment is good, a transmission method which can transmit data at a high speed such as quadrature amplitude modulation (QAM) for transmitting a few information bits in one transmission symbol or multiple phase shift keying (MPSK) is employed for modulation, and when the wireless communication environment is poor, a transmission method which is resistant to interference noise but has a low data rate such as binary phase shift keying (BPSK) is used for modulation.
In order to adaptively change modulation and coding according to variations of the channel environment, it is necessary to transmit channel environment information from a receiver for estimating the channel environment in the above-described modulation methods.
In addition, a mobile communication system for packet transmission uses hybrid automatic repeat request (HARQ).
The HARQ combines automatic repeat request (ARQ) and forward error correction (FEC) so as to improve reliability and throughput in the data transmission of the communication system.
When an error is generated in received information due to the channel environment, the ARQ requests repeat of the same information until the information does not have an error, thereby improving reliability. The FEC uses an error correcting code to correct an error generated due to the channel environment. In case the channel environment maintains so good a condition that frequency of errors can be reduced, the ARQ can control it. However, when the channel environment is deteriorated, frequency of errors is increased in the received information and a number of repeat requests is increased, which results in a low throughput of the system. Accordingly, the HARQ that combines the ARQ and the FEC has been suggested.
Incremental redundancy (IR) is one of the HARQ. A transmitter generates a sequence encoded with an arbitrarily low data rate from data bits and then transmits a portion of the sequences to the receiver. Whenever the receiver requests the transmitter to retransmit the sequence, the transmitter further transmits only redundant bits. Accordingly, the receiver combines the previously transmitted portion of the sequence and the redundant bits, and decodes the combined bits. As a result, the IR can obtain gains due to the combination, and adaptively reduce redundancy according to the channel environment by gradually decreasing the coding rate in every repeat.
A system that requires high-speed data transmission uses time division multiplexing (TDM) for scheduling a terminal for transmitting data in every frame section in consideration of the channel environment and transmitted data. When one terminal is selected to transmit data in every frame section, if an amount of data to be transmitted is small but the channel environment is good,the code efficiency on a Walsh code space is deteriorated. Here, the Walsh code space indicates a set of Walsh codes.
Therefore, efficiency of the system can be improved by employing code division multiplexing/time division multiplexing (CDM/TDM) for transmitting data to another terminal by using remaining Walsh codes, when there are some Walsh codes remained after allocating a portion available Walsh codes to one terminal in each frame section.
As a result, there is required a method for selecting a terminal for transmitting data in the CDM/TDM system having higher resource efficiency than the TDM system, determining an optimal number of Walsh codes for the terminal, and determining an optimal data rate for using the Walsh codes.